External devices that provide wireless recharging for implantable medical devices are subject to various environmental conditions. For instance, the user may change the position of the external device and/or may change the position of other nearby external objects. Such changes in the environmental conditions, particularly changes in orientation or proximity to large metal objects, result in changes on the loading of a coil that is in use by the external device to emit the recharge energy. When large metal objects are present in close proximity to the external device, the loading can be drastically affected by movement of the external device and/or the large metal objects.
It is desirable to maintain a high level of efficiency and energy throughput in the recharge process so that power being consumed by the external device is not being wasted and so that the amount of time needed to achieve an adequate recharge is minimized. One manner of having a high efficiency is to have a recharge coil in a high-Q circuit. However, changes on the loading of the coil in the external device as discussed above cause the resonant frequency of a tank circuit that includes the coil to also change. If the frequency at which the tank circuit is being driven strays from the resonant frequency of the tank circuit, which is inevitable due to changes on the loading of the coil that cannot be predicted, then the efficiency of the power transfer to the implantable medical device plummets in such high-Q circuits.
Furthermore, when the frequency at which the tank is being driven differs from the resonant frequency of the tank circuit, then a substantial phase angle may occur between the voltage being applied to the tank circuit and the current passing through the tank circuit. In such a case, accurately determining the amount of power being driven into the tank requires that the phase angle be known, which complicates the ability to monitor and control the amount of power being provided from the tank to the implantable medical device.